1. Field of the Invention
This invention relates to a liquid developer for use in development of an electrostatic latent image in electrophotography, electrostatic printing and the like.
2. Description of the Prior Art
Heretofore, a liquid developer used for development of an electrostatic latent image in electrophotography and the like has been generally prepared in such a manner that a pigment, dye and the like are kneaded along with a resin, a dispersing agent, a chargeability controlling agent, a solvent and the like and the resulting paste-like kneaded material is dispersed in a highly insulating carrier liquid. The above-mentioned dispersing agent and chargeability controlling agent are very important components which are used for the purpose of improving the chargeability, dispersion stability and preservation stability of the toner particles in the liquid developer. As such dispersing agent and chargeability controlling agent, linseed oil, soybean oil, rosin, asphalt and the like have been so far used. However, these natural products are not always constant in their compositions and therefore it has been very difficult to prepare a liquid developer having always a constant property. Further, the liquid developer prepared by using those natural products has had many drawbacks, for example, the dispersibility of the toner particles in the liquid developer becomes poor and the charge polarity of the toner becomes unclear during use and preservation thereof for a long period of time.
As a method of controlling the polarity of the toner in the liquid developer, a method of controlling the charged state of the toner in which a surface active agent is dissolved in the carrier liquid of the liquid developer to cause it to adsorb on the toner has been known, in addition to a method of applying various natural products onto the surface of the pigment particles as mentioned above. As the surface active agent used for the above purpose, there may be mentioned numerous materials, for example, a metal soap such as cobalt naphthenate, nickel naphthenate, manganese naphthenate and the like, a metal alkylbenzensulfonate and a phosphatide such as lecithin, cephalin and the like, but due to various manners of use thereof, their effects are varied. Generally, these surface active agents are those having a lower electric resistance, and therefore they lower the electric resistance of the carrier liquid when they are dissolved in such carrier liquid. Consequently, the surface active agent should be dissolved in the carrier liquid to such an extent that it does not destroy the electrostatic latent image having been formed on a photosensitive member, on account of which the amount of the surface active agent to be added is subjected to strict restriction. In view of this, such method using the surface active agent is considered to be defective in that the surface active agent can not be added to the carrier liquid in an amount sufficient to control the chargeability of the toner in the liquid developer.
The materials having been so far used for the purpose of controlling the toner so as to be negatively chargeable by dissolving them in the carrier liquid are very few, and only lecithin, an alkylbenzenesulfonate, a polyamide resin and the like are known. When these materials are used as a chargeability controlling agent in preparing a liquid developer, a sufficiently high interfacial potential can not be imparted to the toner, and such materials generally change in their qualities owing to the oxidation, for example the contact with air while allowed to stand, the oxidizing function of the other components in the toner composition and the oxidizing function of the ozone and the like generating upon corona charging in operation of a copying machine. Further, the chargeability controlling property of such materials is deteriorated due to the materials dissolving out from the photosensitive paper, transferring paper or electrostatic recording paper at the time of the development. In consequence of this, the image density and sharpness of the developed image are liable to lower, and the liquid developer containing such material has had the drawback that it can not be used for a long period of time.